1. Field of the Invention
The present invention relates to silver catalysts for the oxidation of ethylene to ethylene oxide, and especially to the preparation of catalyst carriers or supports having improved properties such that catalysts comprising the carriers have enhanced utility.
2. Description of the Prior Art
Processes for the production of ethylene oxide involve the vapor phase oxidation of ethylene with molecular oxygen using a solid catalyst comprised of silver on a carrier such as alumina. There have been efforts by many workers to improve the effectiveness and efficiency of the silver catalyst for producing ethylene oxide. U.S. Pat. No. 5,051,395 provides an analysis of these efforts of various prior workers.
Carriers for ethylene oxide catalysts are most often comprised of low-porosity alpha—Al2O3 particles sintered together with the aid of bond materials. The ethylene oxide (EO) catalyst is commonly produced by depositing silver and various activity and selectivity promoters onto the carrier. Deposition can be accomplished in a variety of ways including adsorption, exchange, precipitation or impregnation. Silver and promoters can be deposited sequentially or co-deposited in a single step, or by a combination of sequential or co-deposition steps. Following the deposition step(s), the finshed catalyst is generally obtained by heat treatments such as drying, calcinations or other activation procedures.
Important parameters in evaluating catalyst performance are the efficency for making EO (i.e. EO selectivity), catalyst activity, and catalyst stability. Performance can be influenced by composition of both the carrier and catalyst and the preparation and processing procedures applied to both carrier and catalyst.
Catalyst stability, or resistance to deactivation, can be improved by treating the carrier prior to deposition of silver and promoters. A particularly beneficial pretreatment procedure involves washing the carrier in pure water, or in aqueous solutions containing active ions. The exact mechanism by which increased stability is realized is not clear. However, it is known that during washing, materials are leached from the carrier such as alkali metal cations, alkaline earth metal cations, silica(tes), alumina(tes), aluminosilica(tes), and the like.
Carrier washing generally affords a modest improvement in catalyst stability. However, it remains an objective of workers in the field to make further improvements.